Typically, a vehicle navigation apparatus includes a board called a navigation board. The navigation board is constructed such that a central processing unit (CPU) and peripheral components such as a memory device and a high-frequency element are mounted on a circuit board. The navigation board is mounted in a case together with other devices such as a hard disk drive.
A vehicle navigation apparatus is installed in a vehicle, where the temperature becomes relatively high. In recent years, a vehicle navigation apparatus has become increasingly sophisticated, i.e., a CPU used in the vehicle navigation apparatus become increasingly sophisticated. Accordingly, the amount of heat generated by a CPU has increased. Therefore, a cooling structure for cooling a CPU and peripheral components has become important.
For example, as disclosed in U.S. Pat. No. 6,847,524 corresponding to JP-A-2002-368467, a laptop computer cooling structure has been proposed that includes a heat spreader (i.e., heat sink), cooling fins integrated with the heat spreader, and a fan device for supplying air between the cooling fins. The heat spreader is placed above a circuit board on which a heat generating semiconductor package (i.e., CPU) is mounted. The heat spreader thermally contacts the heat generating semiconductor package.
As shown in FIG. 3, the inventor has applied such a laptop computer cooling structure to a cooling structure for cooling a navigation board. A CPU 2 as a heat generating device and a plurality of circuit elements 3 (peripheral components) are mounted on a circuit board 1. A heat spreader 4 shaped like a plate and made of aluminum is placed above the circuit board 1. A contact portion 5 is integrally formed on a bottom surface of the heat spreader 4 and thermally contacts the CPU 2.
A plurality of cooling fins 6 is integrally formed on a top surface of the heat is placed at the heat spreader 4 and extends in the right and left direction in FIG. 3. A rid 8 such as a metal plate is placed above the cooling fins 6 so that air passages are provided between the cooling fins 6. A fan device 7 is placed at the right corner portion of FIG. 3. As indicated by arrows of FIG. 3, when the fan device 7 is driven, air is withdrawn into the air passages from above, and the air flows through the air passages from left to right in FIG. 3. Thus, heat generated by the CPU 2 is dissipated through the cooling fins 6.
Such a conventional cooling structure is focused on cooling the CPU2, i.e., dissipating heat generated by the CPU2. In short, in the conventional cooling structure, cooling of the circuit elements 3 is not taken into consideration. However, some of the circuit elements 3 have guaranteed temperatures less than that of the CPU 2. Therefore, when the amount of heat generated by the CPU 2 is significantly increased, for example, as a result of sophistication, an ambient temperature of the circuit elements 3 may exceed the guaranteed temperatures of the circuit elements 3.
For example, it is assumed that a guaranteed surface temperature of the CPU 2 is 100 degrees Celsius (° C.), and a guaranteed ambient temperature of the circuit elements 3 is 85° C. In this case, the bottom surface of the heat spreader 4 is required to be cooled below 100° C. For example, the heat spreader 4 is designed so that the bottom surface of the heat spreader 4 is cooled to 95° C. In this case, a heat generating member of 95° C. is located above the whole circuit board 1. As a result, the ambient temperature of the circuit elements 3 may exceed 85° C., which is the guaranteed ambient temperature of the circuit elements 3.
The ambient temperature of the circuit elements 3 can be reduced below the guaranteed ambient temperature of the circuit elements 3 by increasing cooling performance of the heat spreader 4. One approach to increase the cooling performance of the heat spreader 4 is to increase size of the hear spreader 4. However, the increase in size of the hear spreader 4 results in increases in size and cost of the cooling structure. Further the cooling structure shown in FIG. 3 requires the rid 8, which increases the size and cost of the cooling structure.